More make up air questions - rules I guess

alan_s_thefirstFebruary 8, 2013

I posted a little while ago about backdraft coming through my B-vent when I run more than one exhaust fan in the house.

Built in the mid-80's in British Columbia, the house has a 4" duct from outside to the cold air return, call it "make up air" if that's the correct term or fresh air intake. I'm thinking it's inadequate or does not work sufficiently when the fan is not running in the furnace.

An HVAC guy I was speaking with agreed I should probably run a combustion air intake into my mechanical room. Currently I have a mid-efficiency furnace with an older style hot water tank, so the b-vent has two devices.

My question this time is, what do the rules state in terms of make up air? Is the worst case assumed for exhaust fans, ie, all running at once?

I have two bathrooms with exhaust fans, kitchen exhaust vented outside, central vac vented outside, and electric clothes dryer vented outside. Theoretically, if we and house guests had been out in the snow all day, we could all be taking showers, running the dryer and cooking dinner whilst the maid was vacuuming. That would be a lot of negative pressure.

When I did a small test, I could feel warm combustion gases coming back down the b-vent when I had both bathroom and kitchen exhaust running.

I'm guessing a 5 inch combustion air vent would be more desirable, since its larger diameter would create an easier air flow than the B-vent would?

'I would say if you have a make-up air vent as large as your furnance fule, you would be doing great!"

I don't believe that's how it works.

I think for gas burning appliances in a closed space, there's a formula that's probably in the local code. It depends on the BTUs of the appliances. My experience is limited to installations where air vents supply a closed space and the space supplies the appliances, both at atmospheric pressure, but the input vents in that case are quite a bit larger than the exhaust.

The OP's concern is inadequate air supply for his furnace and water heater, not his kitchen exhaust.

OP, as you increase the combustion air supply to the mechanical room itself, the room will have outdoor air and temperatures. The entry to it becomes more like an exterior door than an interior one. A good step is to add a threshold and weatherstripping to the door frame. That might also lead to less back-drafting of your combustion venting when air exhaust devices are in use in the house.

Your fresh air to cold return setup is like leaving a window permanently open - is that what you want?

There's the conundrum - I am not thrilled about the 'open window' system, but I don't see what choice I have.

Currently, the mechanical room doors are 4 louvred bifolds so I'd have to replace them with solid doors before the weatherstripping thing became an issue. The idea of a damper like the Broan one here is appealing:

The Broan device will close a damper when the exhaust fan is off and open it when the fan is turned on. When the damper opens you'll have the exact same outside air supply to the living space that you have now, I don't see why there would be any change insofar as the backdraft condition is concerned.

I'm just a fellow homeowner and not an expert, but it seems to me that If you had better ventilation to and closed up the mechanical room (to isolate it from the rest of the house), the issue would be over.

It's good you were alert enough to recognize the problem, you need a competent person to end your conundrum. I don't think the solution is all that complicated, It may be that a carpenter or handyman is more useful to you than an HVAC guy. Find out what your local codes require before you do anything.

Remember that air is dumb - it doesn't necessarily follow the path you want it to, it follows the one that is shortest and offers the least resistance.